1. Field of the Invention
The present invention relates to an optical fiber sensor which includes a light transmitter/receiver portion, an optical sensor head remote from the transmitter/receiver portion, and an optical fiber optically coupling the transmitter/receiver portion and the sensor head.
2. Discussion of the Prior Art
As a method of measuring a physical quantity, it is proposed to use an optical fiber sensor which includes: a light transmitter/receiver portion having a light source for producing a reference beam of light and a measuring beam of light, and a photosensor which receives the reference beam, and the measuring beam influenced by a subject, for determining the physical quantity associated with the subject; an optical sensor head which changes a transmission parameter of the measuring beam in relation to the external conditions; and an optical fiber for transmitting the reference and measuring beams between the light transmitter/receiver portion and the optical sensor head.
An example of an optical fiber sensor of an optical homodyning type is shown in FIG. 26, wherein a light beam 122 which is generated by a light source 120 is polarized in the "y" direction. The light beam 122 passes a polarizing beam splitter 124, which transmits the y-polarized beam 122 and reflects an x-polarized beam. The y-polarized beam 122 is then focused by a converging lens 126, on the end face of an optical fiber 128. This optical fiber 128 is capable of transmitting a polarized light beam while maintaining the same plane of polarization Namely, the polarization plane of the beam 122 is not changed while it is transmitted through the optical fiber 128. This type of optical fiber is hereinafter referred to as "fixed-polarization-plane optical fiber". The optical fiber 128 is adapted to transmit the incident beam 122 to the sensor head in one of two transmission modes HE.sub.11.sup.x and HE.sub.11.sup.y whose planes of polarization are perpendicular to each other, for example HE.sub.11.sup.y. In this example, the optical sensor head is constituted by a pressure-sensitive, fixed-polarization-plane optical fiber 132 wherein a light beam being transmitted therethrough is phase-modulated according to a variation in the sound pressure. This pressure-sensitive optical fiber 132 is optically coupled to the transmission optical fiber 128 such that the two fibers 128, 134 are rotated about the optical axes by 45 degrees relative to each other. That is, there exists a polarization plane deviation of 45.degree. between the two fibers 128, 132. Therefore, the incoming light beam transmitted through the optical fiber 128 in one of the two transmission modes is transmitted through the pressure-sensitive optical fiber 132 in the two transmission modes. These two modes are reflected by a reflector film 134 on the end face of the optical fiber 132, and each of the reflected two modes is distributed as the two transmission modes HE.sub.11.sup.x, HE.sub.11.sup.y of the transmission optical fiber 128, when the reflected two modes are incident upon the optical fiber 128. Consequently, the reflected two modes transmitted through the pressure-sensitive optical fiber 132 combine and interfere with each other. In this instance, therefore, the optical fiber 132 functions as a polarizer. Although either one of the two transmission modes HE.sub.11.sup.x, HE.sub.11.sup.y from the optical fiber 128 may be received by the polarizing beam splitter 124, the mode HE.sub.11.sup.x is utilized in this specific example of FIG. 26, as an optical signal from the beam splitter 124, so that the optical signal is converted into a corresponding electric signal by a photosensor 136. This electric output of the photosensor 136 represents a change in a selected transmission parameter of the measuring beam, for example, a change in the phase of the beam which is caused by the sound pressure to which the optical fiber 132 is exposed.
In the optical homodyning type of optical fiber sensor discussed above, an automatic adjustment of the initial optical phase of the sensor is necessary in order to provide the sensor with maximum detecting or sensing accuracy. To this end, a mechanical stress is applied to the pressure-sensitive optical fiber 132, so as to establish an initial phase difference or angle of .pi./2 between the two modes HE.sub.11.sup.x and HE.sub.11.sup.y, in the example of FIG. 26. For instance, this stressing of the optical fiber 132 may be achieved by providing the fiber 132 with a piezoelectric element. In this case, however, the piezoelectric element increases the size of the optical fiber 132, and requires means for applying electric power thereto. Alternatively, the initial phase difference of .pi./2 may be established by controlling the wavelength of the beam produced by the light source. However, this alternative method is comparatively technically difficult to practice.
An optical fiber sensor of an optical heterodyning type is illustrated in FIG. 27, wherein a light source 140 such as a horizontal He-Ne Zeeman laser is adapted to produce two polarized beams whose polarization planes are perpendicular to each other, i.e., a measuring beam having a frequency f1 and a reference beam having a frequency f2 which is slightly different from the frequency f1. These beams are transmitted through a fixed-polarization-plane optical fiber 142, and incident upon a beam splitter 144, so that a part of the incident beams is transmitted through the beam splitter 144 while the other part of the beams is reflected by the beam splitter 144. The reflected beams are received by a photosensor 150 through a polarizing plate 146 and a reception optical fiber 148. The photosensor 150 is adapted to detect a reference beam beat frequency f2-f1. f1. Of the two beams which are transmitted through the beam splitter 144, the beam having the frequency f2 is reflected by a polarizing beam splitter 152, passed through a 1/4 wave plate 154, reflected by a mirror 156 and again passed through the 1/4 wave plate, whereby the plane of polarization of the beam is rotated so that the beam is transmitted through the polarizing beam splitter 152 and is received by a photosensor 158 through a polarizing plate 157 and a reception optical fiber 164. The other beam having the frequency f1 which is transmitted through the beam splitter 144 is passed through the polarizing beam splitter 152 and a 1/4 wave plate 160 and reflected by a surface of a subject 162. The reflected beam is again passed through the 1/4 wave plate 160, with the polarization plane being rotated. The beam is then reflected by the polarizing beam splitter 152 and received by the photosensor 158 through the polarizing plate 157 and optical fiber 164. The beam received by the photosensor 158 has a frequency f1.+-..DELTA.f1. This frequency shift is caused by a displacement of the subject 162. Thus, the photosensor 158 receives the beam having the frequency f2 which has not been influenced by the subject 162, and the beam having the frequency f1.+-..DELTA.f1 which has been influenced by the subject 162. The photosensor 158 is adapted to detect a measuring beam beat frequency f2-f1.+-..DELTA.f1. Outputs of the photosensors 150 and 158 are applied to a phase difference detecting circuit 166, so that a phase difference .DELTA.f1 between the reference and measuring beam beat frequencies is detected. This phase difference represents information carried by the received reference and measuring beams, that is, an amount of displacement of the subject 162.
Unlike the optical fiber sensor of the optical homodying, the optical fiber sensor of the optical heterodyning does not require automatic adjustment of the phase. However, the divided parts of each of the reference and measuring beams having the respective beat frequencies are transmitted from the optical sensor head to the photosensors 150, 158 through the two different optical fibers 148, 164.